After switching from PCs and a Windows server to all Macs and a mac server (with the data transferred over), we are experiencing extreme slowness in opening files shared on the server. It sometimes takes 15 mins. for docs to appear in a folder.

Your post is not very complete so it's very difficult to help you. You mention you are now using a Mac server, can you be more specific?? Apple hasn't made servers for several years however they do produce a OS X 10.8.2 Server version which is only available from the Mac App store, is that what you are referring to?

OK, I would recommend contacting either your local AppleStore business solutions or AppleCare and let them know you are running OS X server edition. If your business works with an AASP that provided the systems that would be another suggestion. You will need to be specific, saying the system is slow isn't specific, if you have someone at your site that acts as an administrator I'd recommend then be the first contact point.

Rather than Apple Care or the Apple Store, there are business solutions people in Apple who have the specific experience to deal with business' needs and applications. You should specify that you want to contact one of those individuals. On the apple.com website you can click on "Business Store". You'll get a page with an 800 number for "Apple Business Experts". It is 1-800-854-3680. Those are the people you ought to speak with.